1. Field of the Invention
The present invention relates to computer displays and to liquid crystal computer displays. More particularly, the present invention relates to methods for providing grey scale up to the visible limit for display on liquid crystal displays.
2. The Prior Art
Numerous methods are known for producing grey-scale images on digital (i.e., on/off) displays with characteristics similar to super-twisted nematic (STN) liquid crystal displays (LCD). Modulating individual pixels by employing frame-rate-control (FRC) algorithms over time and by spatial dithering appear to be the preferred methods for realizing gray scale on displays with STN LCD and similar characteristics.
Prior-art methods producing such grey-scale images do not claim gray scale approaching the visible limit, but are usually limited to 16 gray shades when using FRC alone and to 64 gray shades when FRC and dithering are used in combination. The details of particular methods are not readily available in the literature.
These prior-art methods all suffer from one or more shortcomings. The disadvantages of these previous methods may be classified into three main categories: undesirable visual motion artifacts, undesirable visual static artifacts, and reduced gray shade fidelity.
Undesirable visual motion artifacts may be perceived as "shimmer," "crawl," "ripple," or "waterfalling." The terms used to describe these artifacts are descriptive of subjective perception and are not readily quantifiable. All observed implementations of previous methods (particularly FRC-based processes) have specific gray shade values ("sensitive gray shades") for which one of these artifacts is readily perceivable.
These sensitive gray shades will change from one specific STN LCD panel model to another as a result of differences in panel physical characteristics. Because of this variability among different displays, systems employing these displays have to be tuned to achieve the best result given a combination of the gray scale method and the physical display employed. This normally involves changing the specific FRC implementation and may optionally involve remapping from a sensitive gray shade to an adjacent one that is insensitive, i.e., a gray shade which does not have an undesirable visual motion artifact. Remapping sensitive gray shades to avoid undesirable visual motion artifacts also reduces gray shade fidelity and resolution.
FRC-based motion artifacts are caused by the cyclic nature of the FRC as applied over a small physical area. For a given frame, the FRC is used to assign a value to a pixel within a grid or matrix. This value is compared to the gray shade to determine if the pixel should be "on" or "off." If neighboring pixels cycle harmonically in time with respect to each other, the human eye may perceive motion where none is intended. Virtually all visual motion artifacts are undesirable.
Undesirable static artifacts may be the result of sacrificing spatial resolution to achieve increased perceived gray shades by employing a spatial dithering process. The most noticeable static artifact in spatially-dithered grey scale images is "graininess." If a source gray shade cannot be directly represented, the display system may map the gray shade alternately into the two closest gray shades by dithering. The gritty appearance is caused by high frequency transitions between physically adjacent pixels at a physical resolution the eye can distinguish.
Reduced gray shade fidelity can be manifested as "contouring", also known as Mach banding or "cartooning." For example, contouring causes a smoothly shaded sphere to appear as a series of concentric rings each of a single gray shade. In cartooning, an image has a narrow enough gray shade or color range that images appear to have been drawn with crayons for use in cartoons. Contouring and cartooning are all apparent in processes having a capacity of fewer gray shades than the perceivable visible limit, which is 256 grey shades.